The present invention relates to a valve timing control apparatus and method for internal combustion engines.
To optimize the power and fuel economy throughout the entire engine operating range, internal combustion engines, such as automobile engines, that are equipped with a variable valve actuation mechanism for changing the valve timing of intake valves according to the engine operating state have been made practical. For example, variable valve actuation mechanisms are known that are provided with oil from a mechanical oil pump and operate based on the pressure of the oil.
To perform stable combustion while guaranteeing necessary compression ratio during an idling operation of such an internal combustion engine, the valve timing of intake valves is set to timing that guarantees the necessary compression ratio (hereinafter, referred to as idling valve timing).
Also, when an engine starting operation is initiated (when cranking of the engine is started), the compression ratio is preferably lowered to reduce shock. Specifically, it is preferable that the valve timing of the intake valves be further retarded compared to the idling valve timing to the most retarded timing so that the valves are closed at the most retarded timing.
Therefore, after an engine stopping operation is initiated during idling, the valve timing of the intake valves is changed from the idling valve timing to the most retarded timing in preparation for the next initiation of the engine starting operation (refer to Japanese Laid-Open Patent Publication No. 2000-204987).
When the stopping operation of an internal combustion engine is initiated, the pressure of oil discharged by a mechanical oil pump substantially drops to zero in accordance with a reduction in the engine speed, and the power for operating the variable valve actuation mechanism is also reduced. Therefore, in the period from when the stopping operation is initiated during idling to when the engine rotation is actually stopped, the valve timing of the intake valves cannot necessarily retarded to the most retarded timing. If the valve timing of the intake valves are not retarded to the most retarded timing until the engine rotation is stopped, the initiation of the next engine starting operation will be executed with a high compression ratio. Occupants of the vehicle therefore are likely to feel shock when cranking is started.
To deal with such problems, it may be configured that, when stopping an internal combustion engine, the valve timing of the intake valves are changed to the most retarded timing before initiating an operation for stopping autonomous operation of the engine. In this case, however, idling must be continued until the valve timing of the intake valves are changed to the most retarded timing. During idling in this state, the valve timing is more retarded compared to that during normal idling, i.e., idling valve timing. Thus, combustion cannot be performed while guaranteeing necessary compression ratio. This inevitably destabilizes the combustion and decreases the fuel economy.